Drugs that inhibit DNA repair enhance the lethality of antineoplastic agents. Our objective is to investigate the potential application of DNA repair inhibitors to clinical cancer therapy. Recent studies from our laboratory have helped to define the mechanism of action of two types of DNA repair inhibitors, methylxanthine and aminobenzamide analogues, and these data have allowed us to design rational treatment schedules to dramatically improve the cytotoxicity of certain DNA damaging agents. Our studies have also shown differential enhanced lethality in human cancer vs. normal cells in vitro after treatment with alkylating agents, and our animal studies show an improved therapeutic index of combinations using cytoxan with DNA repair inhibitors in experimental therapy of human tumor xenografts. These studies will be designed to provide essential preclinical data for future clinical trials to selectively improve therapeutic results with chemotherapy using nontoxic DNA repair inhibitors. The first specific aim will be to investigate the mechanism of action of DNA repair inhibitors in human cancer cell lines under conditions of enhanced lethality. These treatments will also be studied in normal human cells which are matched to tumor cells by growth rate or histologic type. Methods used will include clonogenic assays for viability, autoradiography and flow cytometry for cell cycle analysis, and chromosome preparations and unscheduled DNA synthesis for analysis of DNA damage and repair. The second specific aim will extend these in vitro studies to design rational therapeutic regimens which will be tested in the "subrenal capsule assay" using human tumor xenografts in mice. These animal studies will provide data related to therapeutic synergism, toxicity, and effective doses and schedules for the design of future clinical trials, and they will determine the most optimal therapeutic regimens using DNA repair inhibitors in various human cancers treated with clinically relevant forms of chemotherapy.